A previous human trial with selenized yeast showed that selenium supplementation significantly reduced the incidence of lung, colon and prostate cancers. An intervention trial using cancer morbidity as the endpoint takes a long time to complete and is very costly. Surrogate intermediate endpoints or biomarkers are therefore needed to evaluate the efficacy of intervention. Our goal in the near future is to plan for a breast trial; a viable approach will be to use selenium-responsive biomarkers which are associated with the molecular mechanism of selenium chemoprevention. The MCF10AT and SUM-190PT human breast cell lines will be used for the proposed research. Upon transplantation, the MCF 10AT cells are able to develop a spectrum of dysplasia that recapitulate the pathology of human proliferative breast disease. Thus this cell line provides an ideal paradigm for studying selenium chemoprevention of high-risk lesions. The SUM-190PT cell line, on the other hand, exhibits amplification and over-expression of two known breast cancer oncogenes: erbB2 and cyclin D1. It harbors a genotype which is very close to that of human breast cancer. Aim 1 will use the Affymetrix GeneChip to identify selenium-responsive biomarkers in human breast cells in an in vitro system. A detailed correlation between the temporal pattern of gene expression changes with distinctive biological outcome would provide important insight into the molecular mechanism of selenium chemoprevention. The modulation of gene products will be confirmed by semi-quantitative RT-PCR and/or Western analysis. Aim 2 will investigate the functional significance of GADD153 in selenium control of cell proliferation and induction of apoptosis. This is a follow-up mechanism study based on our preliminary finding. Aim 3 will verify the expression of the most sensitive biomarkers in a human breast cell xenograft model using SCID mice. Companion studies will be designed to determine the ability of selenium to inhibit the pathologic progression of the xenograft (atypical ductal hyperplasia to ductal carcinoma in situ to invasive carcinoma). The chemoprevention component of the research is an integral part of the biomarker project because it is imperative to assess the biological relevance of the biomarkers as prognostic indicators of breast cancer protection. Aim 4 will study the effect of selenium on the biology and molecular biology of premalignant lesions in the mammary gland of rats treated with a carcinogen. The goal is to broaden our understanding of the significance of clonal suppression of early transformed cells in selenium protection of cancer.